1. Field of the Invention
The present invention relates to an elastic wave device including a piezoelectric thin film, and to a manufacturing method for the elastic wave device. More particularly, the present invention relates to an elastic wave device using a higher-order mode of a plate wave called a Lamb wave, and to a manufacturing method for the elastic wave device.
2. Description of the Related Art
Recently, an elastic wave device using a piezoelectric thin film has been proposed with the development of, e.g., communication apparatuses operating at higher frequencies. For realizing operation at a higher frequency, it is required to increase the acoustic velocity of an elastic wave in a piezoelectric substrate. From that point of view, attention is focused on an elastic wave device using a plate wave with a high acoustic velocity.
The term “plate wave” means an elastic wave propagating through a medium sandwiched between two free boundary surfaces. As described in Mizutani, Toda: “Analysis of Lamb-wave Propagation Characteristics in Rotated Y Cut X-SAW-Propagation LiNbO3 Flat Plate”, The IEICE Transactions A, J68-A, 5, pp. 496-503(1985-05), which is hereinafter referred to as “Mizutani”, an SH (Shear Horizontal) wave and a Lamb wave are known as examples of the plate wave. As further described in Mizutani, the SH wave is a wave containing mainly a U2 component, while the Lamb wave contains mainly U1 and U3 components. A piezoelectric single crystal is an anisotropic crystal. Therefore, the plate wave propagating in the piezoelectric single crystal contains the U1 and U3 components to some extent although it is the SH wave. Similarly, although the Lamb wave contains mainly the U1 or U3 component, it contains the U2 component as well to some extent.
When the plate wave is oscillated, the U1, U2 and U3 components propagate in any mode. In addition, higher-order modes in which displacement of the main component has a plurality of nodes in the direction of thickness also propagate. Assuming that the lowest-order mode without including nodes is a basic mode, the higher-order modes can be discriminated as a secondary mode, a tertiary mode, etc. in ascending order.
On the other hand, Japanese Unexamined Patent Application Publication No. 2002-368576, discloses an elastic wave device using a piezoelectric thin film and utilizing a Lamb wave. In an embodiment described in Japanese Unexamined Patent Application Publication No. 2002-368576, the piezoelectric thin film is made of LiTaO3, and Euler angles of the LiTaO3 are (0°, 126°±20°, 0°). In the embodiment of Japanese Unexamined Patent Application Publication No. 2002-368576, the acoustic velocity of the propagating plate wave is about 3300 m/sec to 4100 m/sec.
As mentioned above, the higher acoustic velocity of the elastic wave is strongly demanded from the viewpoint of realizing the operation at a higher frequency. In the elastic wave device, described in Japanese Unexamined Patent Application Publication No. 2002-368576, utilizing the plate wave, the acoustic velocity is about 3300 m/sec to 4100 m/sec, and it is not sufficiently high. Therefore, when constructing an elastic wave device used in a high-frequency band, i.e., in a band of about 4 GHz, an electrode finger width of an IDT (interdigital transducer) electrode has to be set to about 0.2μm. This causes the problem that a very high level of processing accuracy is required to form the IDT electrode. Another problem is that, because the electrode finger width is very narrow, wiring resistance may deteriorate, or an electric power handling capability may decrease.